1. Field of the Invention
This invention relates to a pressure transducer including a semiconductor pressure-sensitive element, and in particular, a pressure transducer including a pressure-sensitive element based on the piezoresistive effect.
2. Description of the Prior Art
A differential pressure sensing device 10, as shown in FIG. 1, is well known as a pressure measuring device having a semiconductor pressure-sensitive element. The differential pressure sensing device 10 includes a body case 14 in which a pressure transducer 12 having a semiconductor pressure-sensitive element is housed. The semiconductor pressure-sensitive element measures the value of pressure applied by utilizing the piezoresistive effect of a resistance layer which is formed by diffusing an impurity into the opposite surfaces of a semiconductor substrate made of, for example, silicon. In order to apply different pressures between both the surfaces of the pressure-sensitive element, first and second flow passages 16a and 16b are provided in the body case 14. The first flow passage 16a is opened at one end to a first surface side of the pressure-sensitive element and blocked at the other end by a first isolation diaphragm 18a. The second flow passage 16b is opened at one end to a second surface side of the pressure-sensitive element and blocked at the other end by a second isolation diaphragm 18b. Pressure transmitting media 20a and 20b are sealed in the corresponding first and second flow passages 16a and 16b. The values of pressure Pa, Pb (Pa&gt;Pb) to be measured with respect to the diaphragms 18a, 18b are transmitted to the corresponding surface sides of the pressure-sensitive elements. In this way, a stress corresponding to a difference (Pa-Pb) between the values of pressure Pa and Pb acts on the pressure-sensitive element 12. The stress corresponding to the above-mentioned pressure difference is produced in the pressure-sensitive element and the corresponding stress is also produced in the resistive layer of the pressure-sensitive element. The resistive layer has its resistive value varied, by the piezoresistive effect, according to the stress applied thereto. The pressure difference acting on the pressure transducer 12 can be measured by measuring the resistive value of the resistive layer.
The pressure-sensitive element, if applied with a value of pressure exceeding its elastic limit, will be fractured. Now suppose that the values of pressure Pa and Pb to be measured are, for example, 140 kg/cm.sup.2 and 141 kg/cm.sup.2, respectively. In this case, the pressure difference will be 1 kg/cm.sup.2 which is within the measuring range. However, there is a possibility that the first value of pressure Pa only will be produced due to a failure of a valve which is attached to an external introduction line for processing fluid, etc. In this case, a pressure value of 140 kg/cm.sup.2 acts directly on one surface side of the pressure-sensitive element, causing the pressure-sensitive element to be destroyed. In order to prevent the element from being destroyed by an excessive value of pressure, an excessive pressure preventing mechanism 22 is disposed within the body case 14. The mechanism 22 includes a bellows 24 for separating the first and second pressure transmitting media 20a and 20b from each other and a valve stem 26 extending through the bellows 24, and fixed in place. The valve stem 26 has its opposite ends located in the first and second flow passages 16a and 16b, respectively, and includes first and second valve bodies 28a and 28b. The area between the first valve 28a and the pressure-sensitive element in the first flow passage 16a is restricted by a first projection 30a provided on the body case 14, while the area between the first valve 28b and the pressure-sensitive element in the second flow passage 16b is restricted by a second projection 30b provided on the body case 14. First and second O-rings 32a and 32b are provided on the valve bodies 28a and 28b, respectively, so as to block the first and second flow passages 16a and 16b. In this way, the excessive pressure preventing mechanism 22 prevents the direct action of the excessive value of pressure on the pressure-sensitive element. Now assume that, for example, the second value of pressure Pb to be measured is an excessive pressure. In this case, the second isolation diaphragm 18b is pushed to the left in FIG. 1 and thus the bellows 24, first and second valve bodies 28a and 28b and valve stem 26 are moved by the second pressure transmitting medium 20b, as one unit to the left in FIG. 1, causing the second O-ring 32b to firmly abut against the second projection 30b. Movement of the second pressure transmitting medium 20b for transmitting the second value of pressure Pb to be measured is prevented, and thus the excessive pressure is prevented from being transmitted to the pressure-sensitive element. In this way, the excessive pressure preventing mechanism 22 causes only the pressure value not exceeding the elastic limit of the pressure-sensitive element to act on the pressure-sensitive element, permitting it to be completely protected against excessive pressure. However, the provision of the mechanism 22 in the body case 14 requires a bulkier device as well as an increased amount of pressure transmitting medium (20a, 20b). The mechanism 22, however, serves as an indirect means for preventing the application of excessive pressure to the pressure-sensitive element by blocking the movement of the pressure transmitting medium. A growing demand has been made for a direct means from the standpoint of reliability as well as safety.